The present invention relates to microelectronic circuits and pertains particularly to the construction of bumps on conductive pads for microelectronic chips.
Microelectronic circuits may include a number of chips which are connected into the circuit by means of conductors which are connected to bumps on conductive pads on the chip. Connections are made to external sources from a conductive chip by means of electrical conductors such as wire or tape connected to bonding pads. The bonding pads are typically constructed by vacuum evaporating of metal coatings onto a chip. Where tape leads must be bonded to the pads, the pads may be built up by means of a bump built up on the pad and extending upward therefrom. The bumps are aligned with the conductive portion of the pad and produced either by vacuum evaporating or plating of metallic layers over the conductive pad area. The layers are plated on the pad through openings in photoresist layers or the like on the chip.
Because the fundamental object of microelectronic circuits is to make the circuits and elements as small as possible. The conductive pads and bumps must be as small as possible in order not to consume excess space. The bumps are typically built up on the pads to provide a structure to which tape leads are connected by some form of welding or soldering technique. One technique of connection of such leads is by thermocompression bonding. This requires the application of a combination of temperature and pressure to the pad and tape during the bonding process. It has been found that a high number of chips crack in the silicon dioxide layer beneath the pad structure as a result of this bonding technique.
The fabrication techniques presently used result in the base of the bump structure extending outward and overlapping the edge of the bonding pad. It has been found that this prevents the normal thermal expansion of the bonding pad during thermocompression bonding and thus results in cracking of the underlying oxide layer.
Since this cracked oxide may lead to reliability failures, it is desirable that some means be available to prevent the cracking of the oxide layer during the bonding of leads to a pad structure.